1. Field of the Invention
The present invention relates to the testing of molten material, more particularly relates to a method and apparatus which uses differential temperature measurements to determine characteristics such as superheat, alumina concentration and sodium fluoride to aluminum fluoride ratio of an aluminum smelting bath.
2. Prior Art
Aluminum is conventionally produced in a smelting operation in an electrolytic cell of the established Hall-Heroult design. In a conventional Hall cell, alumina is electrolytically reduced to aluminum in a molten electrolytic bath generally composed of sodium cryolite (Na3AlF6) and aluminum fluoride (AlF3) as well as other additives. Alumina (Al2O3) is introduced into the molten electrolyte bath, dissolves and reacts to form carbon dioxide and aluminum that accumulates as molten aluminum pad. Control parameters monitored during an aluminum smelting operation include the temperature of the bath and the composition of the molten electrolytic bath. Typically, samples of electrolyte are periodically withdrawn from the Hall cell and analyzed for the concentration of alumina and the ratio of the concentration of NaF to the concentration of AlF3 (termed the bath ratio) in laboratory batch tests. Such laboratory tests are typically completed several hours or days after the sampling occurs with little indication of current process conditions.
One probe that has been developed to measure the bath temperature and liquidus temperature of an aluminum smelting bath is described in U.S. Pat. No. 5,752,772 and is available from Heraeus Electro-Nite under the commercial designation of Cry-O-Therm. The probe includes a copper cup surrounded by a cardboard tube and a thermocouple extending into the cup. The probe is submerged in the molten bath and a bath temperature reading is taken. A sample of the bath in the cup is removed and allowed to cool. The temperature of the cooling sample is monitored over time. An abrupt change in the slope of the cooling curve for the sample is taken as the liquidus temperature for the bath. The difference between the bath temperature and the liquidus temperature is determined to be the superheat of the bath. The probe has several drawbacks including its limited utility (no ability to measure the bath ratio) and fragility in the Hall cell environment. The temperature probe may be used only once because the copper cup, cardboard tube and thermocouple of the probe are damaged by exposure to the harsh conditions of the smelting bath. In addition, a portion of the molten aluminum pad produced in the Hall cell occasionally rises up into the smelting bath and contacts the temperature probe. Such direct metal contact destroys the probe before temperature readings can even be made. Likewise, when carbon dust accumulates on the surface of the bath, the probe cannot make an accurate temperature measurement.
An apparatus for measuring the bath ratio as well as the superheat of an aluminum smelting bath is disclosed in U.S. Pat. No. 6,220,748, incorporated herein by reference. The apparatus includes a test sensor that measures the temperature of a sample of the bath and a reference sensor which measures the temperature of a reference material. The reference material does not undergo a phase change whereas the test sensor detects the temperature of the sample of smelting bath as it cools and solidifies. The temperature differential between the reference sensor and the test sensor is monitored and analyzed to determine various characteristics of the bath. The NaF:AlF3 ratio and Al2O3 concentration in the bath are determined in order to control smelting of aluminum metal. In addition, the bath temperature and liquidus temperature are measured to determine the amount of superheat in the bath. In the apparatus disclosed in the patent, the reference sensor and the test sensor are positioned at spaced apart locations. It has been found that the accuracy and consistency of the temperature measurements of the spaced apart sensors are insufficient for determining the bath composition.
Accordingly, a need remains for a molten bath testing probe which accurately determines superheat and bath composition in an aluminum smelting bath.